Method for producing a tellurium steel article

ABSTRACT

A method for producing a hot rolled tellurium steel article having an interior with a relatively substantial amount of tellurium dispersed throughout and a skin relatively devoid of tellurium and free of surface checking. The article is hot rolled from an ingot to which tellurium is added after a solid skin has formed on the sides of the ingot mold. When made from rimming steel, the article is not only highly machinable but also readily extrudable.

United States Patent Bernard S. Levy Chicago;

Gary W. Ilenger, Homewood, both of, 111. 802,798

Feb. 27, 1969 July 6, 1971 Inland Steel Company Chicago, 111.

Inventors Appl No Filed Patented Assignee METHOD FOR PRODUCING A TELLURIUM STEEL ARTICLE 7 Claims, 5 Drawing Figs.

US. Cl 29/527.7, 164/58 Int. Cl "B2314 19/00, 823p 17/00 Field of Search 29/5275,

[56] References Cited UNITED STATES PATENTS 2,187,415 l/l940 Daniels 164/58 2,979,793 4/1961 Wilson et al. 164/59 3,414,042 12/1968 Behrens et al. 164/57 Primary Examiner-John F. Campbell Assistant ExaminerD. C. Reiley Attorney-Merriam, Marshall, Shapiro and Klose ABSTRACT: A method for producing a hot rolled tellurium steel article having an interior with a relatively substantial amount of tellurium dispersed throughout and a skin relatively devoid of tellurium and free of surface checking. The article is hot rolled from an ingot to which tellurium is added after a solid skin has formed on the sides of the ingot mold. When made from rimming steel, the article is not only highly machinable but also readily extrudable.

PATENTED JUL 5 I971 INCHES FROM SURFACE IN ll INCH BLOOM w t W S e m M ,0 M Q i BACKGROUND OF THE INVENTION The present invention relates to hot rolled tellurium steel articles which are free of surface checking and to methods for producing such articles.

During hot rolling an ingot has its cross section reduced to form a succession of intermediate articles, such as billets or blooms, having a polygonal cross section; and these intermediate articles are in turn hot rolled to a reduced cross section, the final article usually being a bar.

The machinability of steel is improved by adding tellurium thereto. During the hot rolling of tellurium steel there is a tendency for the steel-to develop a surface defect known as surface checking. This is manifest by a large number of small cracks at the surface of the steel article, particularly along the 'come'rs. Severe surface checking renders the article commercially unacceptable.

The problem of surface checking in tellurium steel is noted -in Heitmann et al. U.S. Pat. No. 3,382,700.

SUMMARY OF THE INVENTION A tellurium steel article produced in accordance with the present invention has a surface which is relatively devoid of surface checking. The article has an interior with arelatively substantial amount of tellurium dispersed throughout and a skin which is relatively devoid of tellurium.

In one embodiment, the article is a steel bar which is both readily machinable and readily extrudable. The machinability of the bar is due to the tellurium content thereof and the extrudability is enhanced by relatively low sulfur, carbon and manganese contents in the bar and because the skin of the bar consists essentially of relatively pure iron.

A method in accordance with the present invention comprises filling at least the greater portion of an ingot mold with molten steel not containing tellurium, permitting a solid steel skin to form on at least the sides of the ingot mold and then introducing tellurium into the molten interior of the steel and dispersing the tellurium therein. After the entire ingot has solidified, it has an interior with a relatively substantial amount of tellurium dispersed throughout and a skin relatively devoid of tellurium.

During subsequent hot rolling of the ingot and of the intermediate articles of reduced cross section, there is no diffusion of tellurium from the interior of the article to the skin; and the resulting hot rolled article of reduced cross section has an interior with a relatively substantial amount of tellurium dispersed throughout and a skin relatively devoid of tellurium. Because of the manner in which the tellurium is distributed throughout the cross section of the article undergoing hot rolling, surface checking during hot rolling is minimized or avoided.

The above-described method is useful with both rimming and nonrimming steels. A rimming steel is one which, during solidification in an ingot mold, evolves gas sufficient to maintain a liquid ingot top until a solid side and bottom rim of substantial thickness has formed. The gas causes agitation in the molten steel interior of the ingot. After solidification, the skin of the ingot is relatively devoid of metalloids (e.g. carbon and sulfur) and consists essentially of pure iron, while the ingot in terior contains a relatively high proportion of metalloids con sistent with the average composition of metalloids in the steel.

Other features and advantages are inherent in the product and method claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating the distribution of tellurium throughout the cross section of a tellurium steel article such as a bloom;

FIG. 2 is a vertical sectional view illustrating an ingot mold in which rimming is occurring;

FIG. 3 is a vertical sectional view of an ingot mold in which a nonrimming steel is undergoing solidification;

FIG. 4 is a cross sectional view of a steel article in accordance with the present invention and on which hot rolling has been completed; and

FIG. 5 is an enlarged fragmentary view of the cross section of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring initially to FIG. 2, there is illustrated an ingot mold 10 filled with a rimming steel having a composition typically consisting essentially of, in wt. percent:

Carbon 0.03-0.l5

Manganese 0.25-0.60

Sulfur 0.025 max.

Phosphorous 0.0l5 max.

Iron balance lngot mold 10 is of conventional size and construction, having a base 27% inches square and tapering slightly towards the top. The pouring temperature for the molten steel is typically in the range 2875-2900 F.

Rimming is allowed to occur within mold 10 until a solid steel skin 11 forms on at least the sides of the ingot mold, as well as on the bottom thereof in the embodiment illustrated in FIG. 2. Skin 11 is substantially devoid of metalloids and consists essentially of pure iron.

After solid skin 11 has formed, tellurium is added to the molten interior 12 of the ingot. The tellurium may be in bar form or in the form of pellets or shot and is added downwardly through the top 14 of the unsolidified molten steel. Internal agitation is inherent in rimming steel, and this agitation is indicated by arrow lines 13. This internal agitation helps disperse the tellurium throughout the ingot's molten interior 12. The average tellurium content of the solidified ingot interior, and of articles rolled from said ingot, is typically in the range 0.030.2 wt. percent.

Following solidification of the ingot, ingot mold 10 is stripped therefrom, and the ingot is subjected to conventional hot rolling procedures to reduce the cross section of the ingot through various intermediate stages, including a bloom stage and a billet stage, to a finished article 15 having a round cross section, as illustrated in FIG. 4. The resulting steel article 15 has an interior 16 throughout which is dispersed a relatively substantial amount of tellurium, illustrated exaggeratively at 17, and has a skin 18 relatively devoid of tellurium. Skin 18 is exaggerated in FIGS. 4 and 5, and is not intended to represent a rim zone.

FIG. 1 illustrates the relative distribution or dispersion of tellurium throughout the cross section of a steel article produced in accordance with the present invention. The particular article depicted in the diagram of FIG. 1 is a bloom having an I I inch width. The tellurium content increases from the surface of the article to the center thereof, indicated by the 5% inch mark on the abscissa of the graph.

Lines 20, 21 represent two examples of the relative distribution of tellurium along the cross section of the article between the surface and the center thereof. The actual percentage of tellurium at any location along the cross section would depend upon the amount of tellurium added to the ingot.

The relative amount of tellurium at a given location typically may be anywhere between lines 20 and 21; and the position of lines 20, 21 with respect to each other, and the shape of line 21, can vary depending upon the part of the ingot from which the cross section in question is rolled. For example, if the cross section in question is rolled from the center of the ingot, the tellurium distribution typically will be somewhere between lines 20 and 21 as shown on FlG. 1; whereas, if the cross section in question is rolled from the bottom of the ingot, the lower line 21 could be further below line 20 than in FIG. 1. The abrupt rise, at 22 in line 21, could also be substantially more gradual.

The relative absence of tellurium in the skin of the ingot and of the intermediate articles of reduced cross section, contributes substantially to the relative freedom from surface checking during hot rolling.

The tellurium is added no earlier than one minute after the ingot mold has been filled with the molten steel, and preferably the tellurium is added at a time between two minutes and five minutes after the ingot mold has been filled with molten steel. The longer the delay between the filling of the ingot mold and the addition of tellurium, the thicker is the skin without tellurium on the ingot and on the resulting hot rolled article.

The minimum delay between the filling of the ingot mold and the addition of tellurium can be determined by the presence or absence of surface checking during hot rolling. If surface checking arises during hot rolling, this is an indication that not enough time has been allowed to elapse between the filling of the ingot mold and the addition of the tellurium. That is, not enough time has been allowed for a solid skin, devoid of tellurium, to form. Increasing the delay between the filling of the ingot mold and the addition of tellurium will overcome this problem.

When finished article or bar 15 is rolled from rimming steel, it has not only enhanced machinability but also improved extrudability. The relatively low carbon content (0.030.l wt. percent) improves the ductility of the steel bar, and the relatively low sulfur content (0.025 wt. percent max.) improves the plasticity of the steel bar, both enhancing the extrudability of the steel bar. The low sulfur content permits a lower manganese content (0.25O.6O wt. percent) then would normally be possible if the sulfur content were high. (A relatively high sulfur content requires a correspondingly high manganese content to offset hot shortness normally caused by sulfur). The lower manganese content permits the steel to rim. Because the steel has rimmed, the skin of the steel consists essentially of relatively pure iron which is more ductile than a skin containing substantial amounts of metalloids, and the ductile skin enhances the extrudability of the bar. Although the sulfur content is relatively low, the drop in machinability otherwise resulting from a reduced sulfur content is offset by the tellurium content of the steel (0.03-0.2 wt. percent).

A bar having the composition referred to in the preceding paragraph is the preferred embodiment from the standpoint of extrudability and machinability. Permissible compositions of an extrudable, machinable bar 15, in accordance with the present invention, could fall within the following ranges of elements, in wt. percent:

Carbon 0.030.l8

Manganese 0.25-0.90

Sulfur up to 0.13

Phosphorous 0.04 max.

Tellurium 0.03-0.20

lron essentially the balance An embodiment using a nonrimming steel is illustrated in FIG. 3. An ingot mold 30 is filled about 85-90 percent full with molten steel not containing tellurium. A typical composition for such a non rimming steel would be as follows, in wt. percent:

Carbon 0.07-0.09

Manganese 0.95-1.05

Sulfur 0.28-0.32

Phosphorous 0.05-0.07

lron balance lngot mold 30 is partially filled to level 33, and a solid skin 31 is allowed to form on at least the sides of the mold and, as shown in H6. 3, also along the bottom. After solid skin 31 has formed, the empty uppermost mold portion 34 is filled with molten steel, and tellurium is added to the ingot mold at the same time. In case a solid crust has formed at level 33, the crust is broken before the tellurium is added. The tellurium can be thrown into a descending stream of molten steel and the action of the descending stream of molten steel will disperse the tellurium throughout the molten ingot interior 32. After the tellurium has been added, the average tellurium composition of an article rolled from the ingot would be in the range 0.03-0.20 wt. percent.

Another method of producing an article from a nonrimming steel is to fill the ingot mold, as in FlG. 2, allow a solid skin to form along at least the sides of the ingot mold, and then introduce the tellurium into the interior of the steel by mechanically propelling it downwardly into the ingot mold and then stirring the molten interior mechanically or with compressed inert gas.

lf necessary, ingot mold 30 can be provided with a conventional hot top.

An article produced from nonrimming steel, in accordance with the methods described above, will have a tellurium dispersion similar to that illustrated in FlG. 1, and will have a cross section similar to that illustrated in F I05. 4 and 5. Because the hot rolled article composed of nonrimming steel is relatively devoid of tellurium in the skin thereof, surface checking during hot rolling of the article is minimized.

The present invention is not limited to rimming steels and nonrimming steels nor to the specific compositions of tellurium steel set forth above. For example, other molten steel base compositions to which tellurium (e.g., 0.03-0.2 wt. percent) may be added, in accordance with the present invention, include the conventional A.l.S.l. 1000, 1,100 and l,200 series of carbon steels having the following respective typical wt. percent ranges of elements:

Not only carbon steels but, also, alloy steels may be produced in accordance with the present invention. The important consideration is the addition of tellurium (e.g., 0.03- 0.20 wt. percent) to a molten base steel composition after a solid steel skin has formed on at least the sides of the ingot mold thereby enabling the production of a hot rolled steelarticle having an interior with a relatively substantial amount of tellurium and a skin relatively devoid of tellurium and substantially free of surface checking, i.e., free of such surface checking as would render the article commercially unacceptable.

Other machinability increasing elements may also be added to the steel. These include lead, added in the conventional manner; and typical compositions containing both tellurium and lead are disclosed in Holowaty U.S. Pat. No. 3,152,889. Selenium may be used interchangeably with tellurium and is added in the same way as tellurium, i.e., into the molten base steel composition after a solid steel skin has formed on at least the sides of the ingot mold.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What we claim is: l. A method for reducing surface checking of telluriumcontaining steel during hot rolling, said method comprising the steps of:

filling at least the greater portion of an ingot mold with molten steel not containing tellurium; permitting a solid steel skin to form on at least the sides of said ingot mold while maintaining the interior of the steel in a molten condition;

then introducing tellurium into the molten interior of the steel, through the top thereof, and dispersing said tellurium in the molten interior;

then permitting the molten interior with the tellurium dispersed therein to solidify, thereby forming a totally solid ingot having an interior with a relatively substantial amount of tellurium dispersed throughout and a skin on the sides thereof relatively devoid of tellurium;

and then hot rolling said ingot into an elongated article of reduced cross section having an interior with a relatively substantial amount of tellurium dispersed throughout and a skin relatively devoid oftellurium;

whereby the minimization of tellurium in said skin of said ingot and of said article during hot rolling minimizes surface checking of said tellurium-containing steel.

2. A method as recited in claim 1 wherein:

the delay between said filling step and said tellurium-introducing step is sufficient to assure that said hot rolled article of reduced cross section has a skin relatively devoid of tellurium.

3. A method as recited in claim 1 wherein:

said molten steel has a rimming composition;

said molten steel is allowed to rim in said ingot mold;

said tellurium is introduced while said molten steel is rimming;

and the internal agitation in said molten steel, inherent in said rimming, disperses said tellurium throughout the interior of the molten steel.

4. A method as recited in claim 1 wherein;

a minor portion of said ingot mold is filled with molten steel after said solid steel skin has formed;

and said tellurium is introduced into the ingot mold at the same time as said minor portion thereof is filled with molten steel;

said tellurium being dispersed into the interior of the molten steel in the ingot mold by the action of the addition of said minor portion of molten steel.

5. A method as recited in claim 1 wherein:

said tellurium is introduced into the interior of the molten steel in the ingot mold by propelling the tellurium downwardly into the ingot mold;

and the interior of the molten steel in the ingot mold is then stirred.

6. A method as recited in claim 1 wherein:

said tellurium is introduced into the interior of the molten steel no earlier than one minute after at least said greater portion of the ingot mold has been filled with said molten steel.

7. A method as recited in claim 6 wherein:

said tellurium is introduced at a time between 2 and 5 minutes after at least said greater portion of the ingot mold has been filled. 

2. A method as recited in claim 1 wherein: the delay between said filling step and said tellurium-introducing step is sufficient to assure that said hot rolled article of reduced cross section has a skin relatively devoid of tellurium.
 3. A method as recited in claim 1 wherein: said molten steel has a rimming composition; said molten steel is allowed to rim in said ingot mold; said tellurium is introduced while said molten steel is rimming; and the internal agitation in said molten steel, inherent in said rimming, disperses said tellurium throughout the interior of the molten steel.
 4. A method as recited in claim 1 wherein; a minor portion of said ingot mold is filled with molten steel after said solid steel skin has formed; and said tellurium is introduced into the ingot mold at the same time as said minor portion thereof is filled with molten steel; said tellurium being dispersed into the interior of the molten steel in the ingot mold by the action of the addition of said minor portion of molten steel.
 5. A method as recited in claim 1 wherein: said tellurium is introduced into the interior of the molten steel in the ingot mold by propelling the tellurium downwardly into the ingot mold; and the interior of the molten steel in the ingot mold is then stirred.
 6. A method as recited in claim 1 wherein: said tellurium is introduced into the interior of the molten steel no earlier than one minute after at least said greater portion of the ingot mold has been filled with said molten steel.
 7. A method as recited in claim 6 wherein: said tellurium is introduced at a time between 2 and 5 minutes after at least said greater portion of the ingot mold has been filled. 